Field of the Invention
The present invention relates to an image processing apparatus that generates a synthesized image of an input image obtained from an image sensor including a pixel usable in image pickup and focus detection.
Description of the Related Art
Examples of conventional dynamic range extension methods using pupil division images include a technique disclosed in Japanese Patent Laid-open No. 2002-199284 for acquiring a plurality of images captured at different exposures with different cumulative times or readout gains by different divided photoelectric converters and synthesizing the images to achieve a dynamic range extension. In a dynamic range extension mode disclosed in Japanese Patent Laid-open No. 2002-199284, configuring the divided photoelectric converters to independently output images enables non-additive readout of two images from the photoelectric converters captured with different cumulative times, followed by synthesis of the images at a signal processor.
Japanese Patent Laid-open No. 2013-125095 discloses in a description of the concept of pupil division at an image sensor that a pupil intensity distribution differs for each pupil division image.
However, in the technique disclosed in Japanese Patent Laid-open No. 2002-199284, a more largely extended dynamic range requires a larger cumulative time difference or a larger readout gain difference. The larger cumulative time difference results in different degrees of hand shake between images, leading to an unnatural synthesized image including regions with and without visible blur due to hand shake. The larger readout gain difference results in different degrees of noise between images, leading to an unnatural synthesized image including regions with and without visible noise. Thus, in both cases, an unnatural image including regions with and without visible image degradation is generated.
When the two images captured at different exposures in Japanese Patent Laid-open No. 2002-199284 are provided with a shading correction based on the pupil intensity distribution as disclosed in Japanese Patent Laid-open No. 2013-125095, a large shading contrast-unevenness level difference requires multiplication with a large shading correction gain, which results in noise degradation. This will be described later in detail.